Furthermore, please be advised that Roku Express is only compatible with wireless b/g/n networks on the 2.4 GHz spectrum. Ensure that your router is broadcasting a 2.4GHz Wi-Fi network for the device to connect to.
You can also try configuring your router and use wireless channels 1, 6, or 11 which are the preferred channels in North America. Each time you change the wireless channel, you will likely need to apply/save changes before you are prompted to restart the router.
I am having the same issue. It started with insufficient power first. And then to not connecting to modem or router wifi when the insufficient power screen would NOT come on. I have restarted all units in line several times, power is to the wall. Modem is 2.4, router is both 2.4 and 5. I have tried private network info to connect directly to modem at 2.4 and the Roku device will not find it. All was working fine on the evening of 12/31/21, watched a show with no issues. Went to Roku home page to switch apps and the insufficient power came on? It has been nothing but restarting, trying different cables etc... etc... What is the issue???